This invention relates generally to a digital xerographic printers and copiers, and more particularly to an improved method for estimation of toner usage in such devices. The invention also relates to the control of toner additions to a developer housing based on the estimation method.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
In digital xerographic printers and copiers, the process control software has available the actual number and distribution of pixels in every image. Pixel-counting can and has been used to estimate the amount of toner used in developing a given image. The estimated value representing the amount of toner consumed is used for controlling the addition of toner to the developer housing.
To maintain print quality over the course of a job, toner concentration must be maintained during the job. This usually means adding toner to the developer housing in a controlled fashion during the entire run. In a digital xerographic engine, the number of pixels printed can be roughly correlated to the amount of toner to be used, and hence the amount of toner which should be dispensed to maintain proper toner concentration. When using simple pixel counting, the area of pixels developed is taken as fully developed, that is, toner mass developed can be calculated according to the following equation: EQU M.sub.t =N.sub.p *A.sub.p *DMA.sub.max
where:
M.sub.t is the mass of toner developed PA1 N.sub.p is the number of pixels developed PA1 A.sub.p is the area of each pixel PA1 DMA.sub.max is the maximum Developed Toner Mass Area which can be developed.
Due to the spreading of a digital image, the area of the pixel is not developed fully and some areas outside of the pixel are developed. Neither of these phenomena are accounted for in the forgoing equation.
The error in the equation occurs because it fails to account for the departure of the developed image from the ideal shape (i.e. square or rectangular). This error is greatest when developing isolated pixels, and decreases as the size of a pixel cluster increases. The error can be graphically represented by the deviation from the ideal. It is noted that the simulation data used and disclosed hereinafter reflects one particular xerographic system and setup. For this system, small dots are underestimated but for other systems (e.g. line spreading systems or systems with edge enhancement) the small dot estimation may be different. In any case, the largest errors are for isolated pixels and small clusters of pixels.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, should provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 3,409,901 discloses a xerographic system in which a toner concentration control system feeds toner to the developing mechanism in proportion to the area and density of the print. A cathode-ray tube (CRT) is used to expose a photoconductive member, and the signal which drives the CRT is also provided to a toner feed signal means where the signal is summed . When the signal exceeds a predetermined level an output signal is generated to cause toner to be dispensed into the developer mechanism .
U.S. Pat. No. 4,065,031 describes a device for regulating the dispensing of toner particles to a developer mix . During the operation of an electrostatographic printing machine a sensing mechanism, including a photosensor for determining the density of toner developed on a photoreceptor, outputs signals indicative of the toner concentration . The signals are summed and processed to determine if additional toner should be added to the developer mix.
U.S. Pat. No. 4,721,978, the relevant portions of which are hereby incorporated by reference, discloses an apparatus for controlling the concentration of toner particles used to form a highlight color document . Three signals are generated and processed to regulate the dispense rate of toner particles used to form the highlight color portion of the output document . The first signal is an indication of the percentage of the document area arranged to have color highlighted portions thereon . The second signal corresponds to the rate of toner particle usage per document, as determined by a central processing unit, and the third signal indicates the number of copies to be produced . To determine the amount of highlight color toner used, the three signals are multiplied, the product of the signals being used as a control signal which corresponds to the required dispense rate.
U.S. Pat. No. 4,847,659 describes an electrostatographic machine which replenishes toner in a developer mix in response to a toner depletion signal which represents the toner usage rate . The toner depletion signal is determined from the number of character print signals applied to a print head, or in other words, the number of pixels to be toned. The depletion signal is used in conjunction with a second signal, which represents a proportional toning contrast, such that the constant of proportionality between the toner depletion signal and a toner replenishment signal is adjusted according to the second.
U.S. Pat. No. 4,908,666 teaches a toner replenishment control structure which operates in one of two control states to control contrast characteristics when using developers having two developer materials . The first developer material exhibits contrast characteristics which vary with concentration and the second developer material does not exhibit contrast variation due to concentration variance. The system has a first control state for replenishing the first developer material as a function of a concentration signal and a second control state for replenishing the second developer material as a function of a contrast signal.
Xerox Disclosure Journal, Vol. 6, No. 6 (Nov./Dec. 1981) describes a toner dispensing control system that relies upon an intensity signal, representing the intensity of light reflected from the surface of an original document, and a developed density signal to produce an error signal. Subsequently a combination signal is produced as a function of the error signal, in accordance with a predetermined algorithm, to control the dispensing of toner to the developer material.
U.S. Pat. 5,204,699 describes a method to estimate toner use by summing the intensities of the pixels being imaged. This sum of pixels is then related to an amount of toner to determine the amount to dispense. This allows for the pixel counting routine to count intensity information rather than just binary pixel summing.
U.S. Pat. No. 5,349,377 discloses an improved system for more accurately estimating consumption of toner imaging material in a digital xerographic printer in relation to a count of the digital pixels generating the various images being printed, where the frequency rates of the switching between print and non-print pixels are analyzed to provide weighting factors corresponding to different types of images being printed which affect the consumption of imaging material by the printer, and the pixel counts are weighted by these weighting factors to provide an imaging material consumption calculation based on image types as well as image pixel counts. The pixel count weighting factor is automatically substantially increased for the higher print/nonprint rates, or pixel on/off frequencies, and higher toner consumption by fringe field development, corresponding to halftone images in comparison to solid area images. The pixel count weighting factor is intermediately increased for intermediate imaging frequencies corresponding to normal line text.